I'll second that, at least for domestic flights since it appears from another thread that they're using the "unforeseen" nature of mechanicals to avoid EU required compensation for delays/cancellations.

Being at the origination of your return itinerary allowed US to deny you accommodations, per their policy on delays/cancellations. As others have said, it sure seems like you were effectively denied boarding if you weren't given the option of taking the booked flight or accepting the reroute, which opens up a different avenue for compensation under the IDB policy.

Jim